oh right, i think i get what you mean now!

for your first query you can do as perfecto said, and select an object (in object mode) and hit f on your keyboard (that will focus you on whatever is selected, it also changes the camera's point of interest to that object to, so when you rotate (orbit?) the camera it will rotate around that object

you can move the camera by holding down alt and middle mouse button.

if you haven't done so already then check out the Zoom, pan and roll: Navigation essentials video... it appears in the dialog that first pops up when you start up Maya
if you had clicked the 'do not show this at start up' checkbox then just click Help > Learning Movies

if you didn't know how to navigate around your scene then i'll assume you moved away from the center (origin) of your scene by fluke



in addition to Perfecto's edit note you can tap space bar anytime and you'll switch between the last two views. By default it's the enlarged perspective view and the four pane view